Printed images may contain printing defects, and this is a known problem associated with printing processes. Examples of printing defects include scratches, spots, missing dot clusters, ink smears, streaks, and band defects.
Band defects (also known as mechanical bands) are visually noticeable tone fluctuations that usually appear as horizontal or vertical stripes across a printed sheet of paper (page). These bands, and other printing defects, can be caused in printing processes as a result of process speed variations, machine vibrations, drum impact, and other mechanical, physical, chemical, environmental, and algorithmic instabilities. Printing defects are undesirable as they can degrade the perceived quality of the prints. Therefore, there is a requirement to evaluate the severity of printing defects such that corrective measures may be taken to improve the quality of the prints. Furthermore, it is desirable to be able to identify bands and in particular the most problematic bands, so that the cause of these bands may be identified and corrected as a priority.
The severity of bands is currently evaluated manually, by human observers. However, this evaluation is subjective since different observers will have contradicting opinions regarding the severity of the same bands. In addition, human observers' opinions are not reliable and reproducible because many different factors will affect the way humans capture band severity. As a result, a committee of human evaluators is used to define the relative perceptual severity of the bands. In a large committee, conflicts between the opinions of the different observers cancel out, giving a more stable result. However, gathering a committee is often not practical and, as a result, evaluation may typically be carried out by just a few human observers.
An additional disadvantage is that evaluating bands is a difficult, subjective task for the human observer. In addition, it takes the evaluation of many sheets to characterize the state of the printing process, this evaluation task is also time consuming and tedious. Furthermore, in real time applications, such as diagnostic routines which automatically identify problems in a printing press, including identifying the source of a print quality problem on the press, manual evaluation of bands is not practical.